FR 2 721 662 already discloses a positive-displacement pump depicted in FIGS. 1A and 1B.
FIG. 1A illustrates the pump of the prior art empty and at rest. The positive-displacement pump 10′ comprises a pump body 12′ and a feeder 16′ comprising a worm 42′. The worm 42′ is able to rotate with respect to the body 12′ about a main axis X′.
The positive-displacement pump 10′ comprises a rubber pressurizing chamber 14′ and two rubber metering chambers 18′, 20′ communicating with the pressurizing chamber 14′. Each metering chamber 18′, 20′ opens into a rubber flow passage 26′, 28′ which, in turn, opens into an intake chamber 32′ of a rubber distribution member 30′.
The pump 10′ also comprises two pistons 60′, 62′ able to move in the metering chambers 18′, 20′ between a top dead center and a bottom dead center with a cyclic reciprocating movement.
FIG. 1B illustrates the pump of the prior art containing rubber and in operation. When one of the pistons 60′, 62′ is at the top dead center, the rubber pressure in the pressurizing chamber 14′ rises. Because of the reciprocating movement of the pump, the metering chambers 18′, 20′ are full at instants that cannot be predicted and that vary according to the fluidity of the rubber. This results in very great variations in pressure in the pressurizing chamber 14′.
The pressure is applied axially in the chamber 14′ firstly to the worm 42′ and secondly to part of the pump body 12′ that is positioned axially in line with the worm 42′. This pressure is schematized by arrows in FIG. 1B. As a reaction to this increase in axial pressure, the pump body 12′ becomes axially elongated by a length L1, whereas the worm 42′ becomes axially compressed by a length L2.
The axial elongation L1 of the pump body 12′ leads to a variation in the volumes of the metering chambers 18′, 20′ and of the flow passages 26′, 28′. Further, the axial elongation L1 of the body 12′ leads to an offset between the intake chamber 32′ and each flow passage 26′, 28′. Because rubber is not very compressible, the variation in these volumes and the offset therefore lead to a variation in the pressure and flow rate on the outlet side of the pump 10′.